In the first place, a brief description will be given as to how the present invention method has proposed, with reference to FIG. 1 illustrating a so-called vertical packaging apparatus, which is one typical type of form-fill-seal packaging apparatus. In the apparatus illustrated, a web 14 of plastic material such as polyethelene sheet is fed from a supply roll 16 of the material over a former or web forming device 12 mounted on the upper part of a vertically extending cylindrical, hollow mandrel 10. At the former 12 the web 14 is formed into tubing, the tubing being fed downward a predetermined length and sealed by a center sealing device 18 so as to form a longitudinal seal on the edges overlapping in the longitudinal direction and sealed at package length intervals. Articles W are supplied from upstream direction into the tubing. Then the tubing is fed forward a predetermined length, intermittently stopped, sealed by an end sealing device 20 so as to form transverse seals and ultimately cut by a cutter 22 into individual packages filled with articles (FIG. 3). The web 14 is intermittently pulled from the supply roll 16 and fed forward toward the former 12 means of by a pair of web feeding rolls 24 located between the supply roll 16 and the former 12. The feeding rolls 24 are intermittently driven for feeding the web 14 forward one package length increment in a given time interval.
The increment of the web 14 fed forward by the web feeding rolls 24 is taken up and pulled over the former 12 under tension by tubing feeding means indicated generally at 26 in FIGS. 1 and 3 below the former 12. The tubing feeding means 26, as illustrated in FIG. 2, comprises a pair of endless belts each designated 30 and disposed on opposite sides of the mandrel 10. Each of belts 30 is trained around a pair of pulleys 28 arranged in vertical alignment at the respective side of the mandrel 10. The belts 30 are rotated in a predetermined direction for feeding the web 14 downwardly so as to be formed into tubing at the former 12 and thence gripped in sealing engagement with the outer periphery of the mandrel 10. In FIG. 1, reference numeral 31 designates a register mark sensor for sensing register marks printed on the web 14. Reference numerals 33 and 35 designate a guide plate and turning bars, respectively.
In the operation of form-fill-seal packaging machines of the type just described, the web 14 may slacken betweeen the web feeding rolls 24 and the tubing feeding means 26, and tends to meander when fed forward toward the former 12. Because of such unsmooth feeding of the web 14, therefore, the machine may have to be operated at a lower package production rate than desired. Additionally, the web feeding rolls 24 intermittently pull a package length increment of the web 14 from the supply roll 16, thereby diminishing the diameter and hence the mass of the supply roll 16 with the passage of time. Thus the resulting augmented pull tends to excessively overrun the supply roll 16, causing the web 14 to slacken in the path from the supply roll 16 to the web feeding rolls 24. Again, because of such slack, the web 14 may not be fed forward smoothly, reducing the efficiency of the operation.